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The plunger pump is a key component in hydraulic presses; it functions similar to the heart in the human body. Any problems with it can affect the operation of the entire machine, so proper maintenance of the plunger pump helps to prevent failures in the hydraulic press and improves the efficiency of the machine’s operation. The efficiency of a hydraulic system depends primarily on the volumetric efficiency of the hydraulic pump. When this volumetric efficiency drops to 72%, routine maintenance is required, including the replacement of bearings and worn-out seals, as well as the replacement or repair of friction pairs that have exceeded their allowable clearance, in order to restore the system’s performance. Here, the maintenance methods for the straight-shaft swashplate piston pumps, which are widely used in hydraulic presses, are mainly introduced. 1. The oil supply types of piston pumps: Straight-shaft swashplate piston pumps are divided into pressure-fed type and self-priming type. Most pressure-fed hydraulic pumps use a tank filled with pressurized oil; some hydraulic pumps also have a filler pump that supplies pressurized oil to the inlet of the hydraulic pump. Self-priming hydraulic pumps have a strong self-priming capability and do not require external oil supply. For hydraulic tanks that are pressurized with air, it is necessary to wait until the hydraulic tank reaches the required pressure after each startup of the machine before operating it. Using the machine when the air pressure in the hydraulic oil tank is insufficient can cause separation between the components inside the hydraulic pump, as well as abnormal wear on the return plate and the pressure plate within the pump. For plunger pumps that use an oil supply pump, after 3000 hours of operation, the operator must inspect the plunger pump 1–2 times per day to check whether the operating noise of the hydraulic pump is normal. If a decrease in the speed of the hydraulic cylinder or stalling is observed, the oil feed pump should be disassembled for inspection to check for any scratches on the edges of the impeller and to determine whether the clearance of the internal gear pump is too large. For self-priming piston pumps, the level of oil in the hydraulic tank must not be below the lower limit indicated by the oil gauge; an adequate amount of hydraulic oil must be maintained. The higher the cleanliness of the hydraulic oil, the longer the service life of the hydraulic pump. This type of oil pump is widely used in hydraulic presses. 2. Bearings for plunger pumps: The most important component of a plunger pump are the bearings. If there is play in these bearings, it is not possible to maintain the proper clearance between the three friction pairs inside the hydraulic pump. This also affects the thickness of the hydrostatic support oil film surrounding each friction pair, thereby reducing the service life of the bearings in the plunger pump. According to the information provided by the hydraulic pump manufacturer, the average service life of the bearings is 10,000 hours; new bearings are required once this value is exceeded. For the removed bearings, it is impossible to determine their clearance without specialized testing equipment; visual inspection is the only option. If scratches or discoloration are found on the surface of the rollers, they must be replaced. When replacing bearings, attention should be paid to the English letters and model of the original bearings. Plunger pump bearings are usually large-capacity bearings, and it is best to purchase products from the original manufacturer that match the original specifications. If another brand is used, it is necessary to consult someone with experience in bearings to determine the appropriate replacement, in order to maintain the bearing’s precision grade and load capacity. 3. Inspection and repair of three pairs of friction pairs 3.1 Plunger rod and cylinder bore Table 1 shows the replacement standards for piston pump components (see Figure 1). When the various clearances listed in the table are out of specification, they can be repaired using the methods described below: (1) If the cylinder bore is fitted with copper sleeves, these can be replaced as a way to carry out repairs. First, trim the diameters at a set of plunger rods to a uniform size, then polish the outer diameter using sandpaper graded 1000# or higher. Three methods for installing copper sleeves in cylinder blocks: (a) heating the cylinder block for hot fitting, or freezing the copper sleeve at low temperatures for forced fitting through interference fit ； (b) Assembly using Loctite adhesive; this method requires grooves on the outer diameter surface of the copper sleeve ； (c) Tap the cylinder bore, thread the outer diameter of the copper sleeve, apply Loctite, and then screw it in for assembly. (2) For cylinder blocks and copper sleeves bonded by sintering, the repair method is as follows: (a) Use an abrasive rod to grind and repair the cylinder bore manually or mechanically ； (b) Use a coordinate boring machine to re-bore the cylinder block holes ； (c) Use a reamer to repair the cylinder block bore. (3) The “surface engineering technology” is employed, with the following methods: (a) Electroplating technique: Plating a layer of hard chromium on the surface of the plunger ； (b) Brush plating technology: Plating wear-resistant materials on the surface of the plunger ； (c) Thermal spraying or arc spraying or electro-spraying: spraying high-carbon martensitic wear-resistant materials ； (d) Laser cladding: Cladding high-hardness wear-resistant alloy powder on the surface of the plunger. (4) For cylinder blocks that lack copper sleeves, the material is usually ductile iron, and an amorphous film or coating is prepared on the inner wall of the cylinder block. It is because of this special substance on the inner wall of the cylinder bore that a hard-hard friction pair can be formed. If the cylinder hole is ground blindly, removing the surface material from the inner wall of the cylinder hole will change the structural properties of the friction pair. If a friction pair with its coating removed is used forcefully, the temperature of the friction surface will rise sharply, causing adhesion between the plunger rod and the cylinder bore. In addition, a unique thin film coating is applied to the surface of the plunger rod; this coating possesses anti-friction, wear-resistant, and lubricating properties. In this type of friction pair, the materials involved are actually hard and soft. If the coating is altered artificially, it disrupts the optimal pairing of materials for friction. To repair such special plunger pumps, they must be sent to a specialized repair shop. 3.2 Sliding friction between the slip shoes and the swash plate: The sliding friction between the slip shoes and the swash plate is the most complex of the three friction pairs in a swash plate piston pump. Table 1 shows the clearance between the plunger rod ball head and the swash plate socket (see Figure 2). If the clearance between the plunger and the swash plate is out of spec, the high-pressure oil in the plunger chamber will leak through this clearance. This leads to a thinning of the oil film between the swash plate and the inclined disc; in severe cases, it can result in the failure of the hydrostatic support. Metal-to-metal contact and friction occur between the swash plate and the inclined disc, causing the swash plate to abrade and detach, as well as the plunger ball head scratching the inclined disc. When the ball head of the plunger rod and the socket of the slider shoe exceed the tolerance by 1.5 times, they must be replaced as a set. Standard clearance for the plunger rod and cylinder bore: φ16 – 0.015; φ20 – 0.025; φ25 – 0.025; φ30 – 0.030; φ35 – 0.035; φ40 – 0.040. Maximum clearance: 0.040 for φ16, 0.050 for φ20, 0.060 for φ25, 0.070 for φ30, 0.080 for φ35, 0.090 for φ40. Standard clearance for the plunger rod ball head and slider socket: 0.010 for φ16, 0.010 for φ20, 0.015 for φ25, 0.015 for φ30, 0.020 for φ35, 0.020 for φ40. Maximum clearance: 0.30 for φ16, 0.30 for φ20, 0.30 for φ25, 0.35 for φ30, 0.35 for φ35, 0.35 for φ40. After the swash plate has been in use for some time, its surface may become concave; before performing planar grinding, it is necessary to first measure the original dimensions and surface hardness. After grinding, measure the amount of grinding; if it is within 0.18, it poses no issue for use in piston pumps ； If it exceeds 0.2 mm, nitriding should be used to maintain the original thickness of the nitrided layer. When grooves are formed on the swashplate surface by scraping from the plunger ball head, repair can be carried out using laser cladding of alloy powder. Laser cladding technology can ensure both the bonding strength of the material and the hardness of the clad material, without reducing the hardness of the surrounding tissues. Chromium-phase welding rods can also be used for manual surfacing; the repaired swashplate surface requires re-heat treatment, preferably using a nitriding furnace. Regardless of the method used to repair the swash plate, its original dimensional accuracy, hardness, and surface roughness must be restored. 3.3 Repair of the valve plate and the cylinder block’s valve distribution surface